In the darkest regions of deep space, the temperature is a chilly
-450° Fahrenheit. Closer to our Sun, temperatures reach thousands
of degrees Fahrenheit. What makes Earth's climate so moderate? Separating
Earth from the extreme and inhospitable climate of space is a 500-mile-thick
cocoon of gases called the atmosphere.

All planets have an atmosphere, a layer of gases that surrounds them.
The Sun's atmosphere is made up of hydrogen, while Earth's is
made up primarily of nitrogen and oxygen. Carbon dioxide, ozone, and
other gases are also present. These gases keep our planet warm and
protect us from the direct effects of the Sun's radiation. Without
this regulation, Earth could not sustain life.

The structure of the atmosphere

The atmosphere is made up of several layers: the troposphere, stratosphere,
mesosphere, ionosphere, and exosphere. Closest to Earth is the
troposphere. Most of the clouds you see in the sky are found in the
troposphere, and this is the layer of the atmosphere we associate
with weather. Extending up to 10 miles above Earth's surface, the troposphere
contains a variety of gases: water vapor, carbon dioxide, methane,
nitrous oxide, and others. These gases help retain heat, a portion
of which is then radiated back to warm the surface of Earth.

Above the troposphere is the stratosphere, which includes the ozone
layer. The stratosphere extends from about 10 to 30 miles above the
surface of Earth. Ozone molecules, which are concentrated in this
layer, absorb ultraviolet radiation from the Sun and protect us from
its harmful effects.

Thirty to 50 miles above the surface is the mesosphere, the coldest
part of the atmosphere. Above the mesosphere, in a layer called the
ionosphere (also called the thermosphere), things start to heat up.
Temperatures in the ionosphere, which extends about 50 to 180 miles
from the surface of Earth, can reach up to several thousand degrees
Fahrenheit. Beyond the ionosphere is the exosphere, which extends
to roughly 500 miles above the surface of Earth. This is the outermost
layer of the atmosphere, the transition zone into space.

The greenhouse effect

The gases in the atmosphere that help retain heat are called greenhouse
gases. These gases, primarily carbon dioxide (CO2), absorb heat instead
of allowing it to escape into space. This "greenhouse effect" makes
the planet a hospitable place.

However, greenhouse gases can have negative effects, too. Human
activity has increased the amount of CO2 in the atmosphere. Since the 1800s, industrialized
societies have burned fossil fuels such as coal, oil, and natural
gas; these processes all give off CO2. During the past 25 years, the amount of CO2
in the atmosphere has increased by about 8 percent. With more CO2 in the atmosphere, more heat is absorbed and retained, causing global
temperatures to rise.

Some scientists project that by the next century, CO2 levels in
the atmosphere could be twice what they are today, causing a global temperature
increase of about 3 degrees. Three degrees may not seem like much, but
even a few degrees can have serious consequences. Tropical diseases
could increase, since mosquitoes and other disease-carrying insects
thrive in a warmer climate. Sea levels could rise, and coastal cities
such as New Orleans and Washington, D.C., could be battered by storm
surges. Prosperous farmland could dry up and agricultural regions
could shift, wreaking havoc on the global economy.

It is possible that the recent warming trend is due more to natural
cycles of cooling and warming than to human activity. Global climate
change occurs on a scale of tens or hundreds of thousands of years,
but scientists have only begun to study these effects in the last 150 years. Still, most scientists agree that just as climate affects
our lives, we can affect the climate. Just how much we can influence
it remains to be seen.